This invention relates to the production of a novel pozzolanic and/or latent hydraulic supplementary cementitious material, also abbreviated as SCM in the following, and binders which contain said material mixed with cement, in particular Portland cement.
Cement, and in this case especially Portland cement, abbreviated OPC (ordinary Portland cement) in the following, is an important construction material on the one hand, but one that requires large amounts of energy and mineral raw materials to produce on the other hand. Hence there have been efforts for some time to reduce the energy and raw material needs, for example by using by-products and waste products.
Substituting Portland cement clinkers with SCMs is especially well-suited for achieving these goals. On the one hand, SCMs are frequently by-products and waste products and therefore reduce the raw material input. The most commonly used SCMs include granulated blast furnace slag and fly ash. On the other hand, lowering the clinker content in turn lowers the energy requirement for the production thereof, because SCMs require less energy to produce than clinkers.
However, by no means all by-products and waste products are suitable as SCMs. The pozzolanic or latent hydraulic reactivity may not be too low, as otherwise the properties of the construction material created from the cement and SCM will be negatively impacted. For example, calcined clay can only be used as an SCM if it has a high mineralogical purity; ideally consists of only one clay mineral. The aluminium oxide content and the Al2O3/SiO2 ratio should be high. Moreover, activation by calcination requires staying within a narrow temperature window as well as the shortest possible calcination times (down to seconds). Because clay is highly absorptive and very fine, a large volume of liquefier is needed for concrete made out of cement and such a SCM in order to compensate for the increased water demand. Admixtures can be ad- and absorbed on the surface and in the clay interlayers, respectively, which makes it necessary to use larger amounts.
High-quality clays consisting of a few or only one phase are rare in actual practice and therefore too expensive because of the competition with other industry branches. However, with mixtures it is difficult to set an optimum calcination temperature, or to put it another way, the different optimum temperatures for different constituents make it impossible to activate the entire starting material. If the temperature is too low, insufficient volumes will be activated. At somewhat higher temperatures, only those phases that react at these lower temperatures will be activated, which in most cases is still too low a fraction. Although a sufficient fraction will generally be activated at medium temperatures, some fractions of the starting material will have already formed crystalline and therefore inert phases. Although (nearly) all fractions of the starting material will be activated at high temperatures, most fractions will have already formed inert crystalline phases. The various clay minerals have the following optimum calcination temperatures:                Serpentinite 400-500° C.,        Palygorskite 600-800° C.,        Kaolinite 600-800° C.,        Halloysite 600-800° C.,        Pyrophyllite 750-950° C.,        Montmorillonite 800-950° C.,        Illite 800-1000° C.,        Mica 650-1000° C.        
Non-converted phases have an especially high water demand and therefore must be avoided as much as possible. Many starting materials also have too low an Al2O3 content, but considerable amounts of SiO2 and other constituents such as Fe2O3, CaO, MgO, Na2O and K2O. For these reasons, many clays cannot be used economically and in certain circumstances clay-containing or clay-rich materials therefore have to be dumped.
It has already been proposed to make such clays usable as SCMs by treating them hydrothermally or by calcining them mixed with limestone or by combining them with limestone; see for example EP 2 253 600 A1 and U.S. Pat. No. 5,626,665. In Tobias Danner's doctoral thesis, “Reactivity of calcined clays”, ISBN 978-82-471-4553-1, it was demonstrated that limestone already present in the starting material or added thereto before burning does not have any influence on the reactivity of the calcined material. It was furthermore established in this study that the material with the highest MgO content originating from magnesium silicate compounds (i.e. not from magnesium carbonate or dolomite to dolomitic limestone) could not be sufficiently activated in order to be used as SCM, in other words had the least pozzolanic reactivity. This study also showed that the lime binding capacity (in other words the pozzolanic reactivity) of the materials studied reaches its maximum at burning temperatures of 700 to 800° C. and that even at temperatures slightly above 800° C., e.g., 850° C., the material loses a substantial amount of reactivity. In other words, higher temperatures led to materials with only very low to even no reactivity at all. Consequently, this method was unable to solve the problems associated with clays with mixed phases, which require very different calcination temperatures. The study furthermore did not reveal any positive effect of the dolomite present in minute concentrations, as the latter had not been added in sufficient quantities and the burning temperatures used were also too low. From this study, a person skilled in the art cannot infer a synergistic effect of the calcination of dolomite to dolomitic limestone in combination with a clay, nor a use of the material thus obtained as an SCM.
Dolomite is another material that cannot be used for cement clinker production, nor as a SCM. MgO can only be incorporated in Portland clinkers in a concentration of up to a few percent; a fraction in excess of that is present in the raw meal as “dead-burned” MgO after burning. Such MgO reacts very slowly, to a large extent years later, with water, but then forms Mg(OH)2, which has a larger volume than MgO and thus destroys the hardened cement. Nor may dolomite be used as an SCM in every case because it partially dissolves, thus releasing CO2 and forming Mg(OH)2 under certain circumstances. The CO2 in turn forms calcite from Ca2+. These reactions likewise lead to a volume change, which can in turn lead to crack formation and destruction of the hardened cement.
An approach for rendering dolomite (and limestone) useful is a burning for direct use as air hardening lime/caustic lime/slaked lime or as a hydraulic binder, e.g. as so-called Roman cement. Various authors have studied the reaction products of calcination of clays with a lime or dolomite content or of mixtures of clay and limestone and/or dolomite, but only with a view towards a use of the products as a hydraulic binder or the production of ceramics. See A. L. Burwell, Mineral Report 28 in “The Henryhouse Marlstone in the Lawrence Uplift, Pontotoc County, Okla. and its Commercial Possibilities” and M. J. Trindade et al., “Mineralogical transformations of calcareous rich clays with firing: A comparative study between calcite and dolomite rich clays from Algarve, Portugal”, Applied Clay Science 42, (2009), pp. 345-355. A suitability as SCMs is not addressed in these works, and comparative studies have furthermore shown that it is not practical for the majority of the products.
Another study on rendering low-quality clay material useful as SCMs also involves an MgO-rich raw material that contains dolomite in traces, see G. Habert, “Clay content of argillites: Influence on cement based mortars”, Applied Clay Science 43 (2009) 322. The predominant MgO fraction is not bound in the dolomite, but present in the form of clay minerals (palygorskite and montmorillonite: Σ 69%). Only a small calculated fraction of less than 1% MgO may be present as carbonate, which corresponds to a maximum amount of 5% pure dolomite. The study also shows that burning temperatures above 800° C. lead to a substantial reduction of reactivity, or rather that the material was only present as an inert filler afterwards.
GB 1438 A proposes the production of a pozzolan from argillaceous materials and calcareous dolomite or magnesian material. The material should be burned at a temperature at which no sintering will take place. The absence of sintering means that the existing compounds such as CaO, MgO or the existing mixture of SiO2 and Al2O3 (see also metakaolin) may not react any further and/or with each other. This becomes necessary in order to prevent the crystallization of new, more complex phases and thereby ensure that the material is as reactive as possible. Adding salts such as sodium chloride should result in decarbonation without sintering, during which synthetic pozzolans will be obtained. The ratio of the argillaceous, dolomitic or magnesium-rich material to the argillaceous constituent should correspond to systems of Roman cements. According to standard practice and definition, the clay fraction is thus 10% to 15%, or at a maximum below 30%.
Another study (I. Barbane et al. 2013, “Low-temperature Hydraulic Binders for Restoration Needs”, Material Science and Applied Chemistry, Vol. 28) describes the production and the material properties of a hydraulic limestone based on dolomite and clay. The goal is to produce a system with a maximum amount of dolomite and the lowest possible clay contents. The strength developing reaction is mainly attributed to the hydration of CaO and MgO for conversion to Ca(OH)2 and Mg(OH)2, and also, but to a lesser extent, to a pozzolanic reaction. According to this document, higher clay contents and correspondingly lower dolomite or limestone contents are not sought, as this would lead to reduced strength development. A combination with, say, OPC is neither indicated nor deemed advantageous by a person skilled in the art because, for example, the hydration of OPC produces large quantities of Ca(OH)2.
Another study (L. Lindina et al. 2006, “Formation of calcium containing minerals in the low temperature dolomite ceramics”, Conference on Silicate Materials, Materials Science and Engineering, Vol. 25) describes the production and use of a hydraulic binder based on natural mixtures of limestone, dolomite, and clay. The study shows that the optimum burning temperature is around 750° C. Reactivity is substantially reduced even at 800° C. For a person skilled in the art, this leads to the conclusion that burning temperatures lower than 800° C. should be sought. A combination with, say, OPC is neither indicated nor deemed advantageous by a person skilled in the art.
In the studies cited, use is made of mixtures with the greatest possible quantity (at least more than 70%, typically more than 80%) of limestone or in rare cases dolomite and only small quantities (less than 30%, typically less than 20%) of clay material. In combinations with OPC, the material produced according to these methods does not lead to an improvement in strength development.
The pozzolanic activity of other natural and synthetic materials which, like pozzolans, contain aluminium silicate, is also (too) low for use as SCMs.
The not prior published document PCT/EP2015/002549 discloses that reactive SCMs can also be obtained from clay, argillaceous material, and low-quality pozzolans that are either not suitable or else poorly suited for other purposes by burning them in combination with dolomite or magnesium carbonate-containing materials. However, the SCMs thus obtained frequently exhibit a pronounced brown or red colouration.
Materials with a greyish or even white colouration are typically used for cement and concrete applications. A pronounced discolouration, for example reddening due to calcined clays or blueness in cements containing granulated blast furnace slag, is often perceived as disruptive. This rules out use in many applications. Hence there is still a need of materials or of methods for the activation of aluminium silicates, in particular of clay and argillaceous materials and other materials of low pozzolanic quality, in order to render them suitable as SCMs.
Surprisingly, it was found that reactive SCMs can also be obtained from clay, argillaceous material, and low-quality pozzolans that are either not suitable or else poorly suited for other purposes, and that a brown or red colouration can be avoided, by burning them in combination with dolomite or magnesium carbonate-containing materials under reducing conditions. In addition, another technical advantage arises from the fact that the decomposition of phases takes place at even lower temperatures under reducing conditions. The material can thus be produced with even greater energy savings.